1. 1 Technical measures to prevent cracking in raw material selection
1. Choose Portland cement (code p .ⅱ Ⅱ) with low hydration heat, alkali content below 0.06%, moderate fineness and strength grade of 42.5, and C3A content in cement should be within the range of 6% ~ 8%. 42.5-grade slag cement (code name P.S.) is selected for mass concrete of bearing platform to reduce hydration heat. It is forbidden to use early strength cement.
2. Adding high-quality ground composite mineral powder and high-quality fly ash into concrete, and the total content is not less than 30%, which reduces the hydration heat, improves the workability of concrete, and at the same time enhances the chloride ion penetration resistance of concrete, preventing concrete cracking and steel bar corrosion caused by hydration heat.
3. Adding efficient water-reducing and retarding pumping agent into concrete can delay the hydration reaction time and speed of cement and reduce the water-binder ratio.
4. In summer construction, in order to further reduce the hydration heat and prevent concrete cracking, citric acid retarder is added while adding high-quality ground mineral powder and Grade I ground fly ash.
5. The total mass of chloride ions introduced by various raw materials in concrete mixture shall not exceed 0.65438 0% (reinforced concrete structure) and 0.06% (prestressed concrete structure) of the total cementitious materials.
6. Strictly control the selection, feeding and inspection of materials, and it is forbidden to use unqualified or problematic materials.
1.2 technical measures for crack prevention in concrete preparation
1. The concrete used in this project is prepared according to the preparation technology of high-performance concrete and constructed according to the construction technology of high-performance concrete, so as to improve the compactness and durability of concrete, reduce the water-binder ratio and the dosage of cementing material as much as possible while meeting the design and construction requirements, and prevent the concrete surface from shrinking and cracking.
2. The initial setting time of concrete should generally not be less than 12h. The initial setting time of pile cap concrete shall not be less than 20h；; When the concrete of pile caps and cast-in-place beams is constructed in summer, a proper amount of citric acid retarder is added to make the initial setting time of concrete reach more than 20 hours. All concrete pouring shall be completed before the initial setting time.
3. On the premise of meeting the technical requirements of concrete design and construction, when preparing concrete, the early elastic modulus of concrete should be reduced as much as possible to avoid the tensile stress caused by the excessive early shrinkage elastic modulus of concrete, which is very important to prevent concrete cracking.
1.3 basic provisions of technical measures for concrete crack prevention
1. Before formal pouring, model trial pouring, trial curing and temperature measurement and control shall be carried out for concrete such as pile caps, pier shafts and cast-in-place beams, so as to finally verify and determine pouring technology, curing methods and procedures, and give reasonable control values of temperature parameters during construction and specific technical measures to prevent cracking.
2. In addition to mixing the concrete mixture in strict accordance with the technical requirements, the slump, expansion rate, air content and bleeding rate of the mixture must be tested in strict accordance with the regulations and controlled within the specified technical indicators to ensure the continuous stability of concrete quality.
3. The concrete into the mold temperature is not more than 30℃. The temperature difference between adjacent new and old concrete is not more than 20℃, and the difference between curing agent sprayed on concrete and concrete surface temperature is not more than 65438 05℃. After 30min, the maximum temperature rise of mass concrete is less than 20℃, and the maximum internal temperature is not higher than 55℃.
4. Before concrete pouring, the surfaces of formwork, steel bars and steel flanges that contact with concrete should be covered with a quilt first, and then with a canvas to control the temperature within the range of 5 ~ 30℃. Spray cooling water on the quilt in summer to cool it to below 30℃. 5. After the completion of cast-in-place concrete pouring, the concrete surface should be covered with a layer of wet linen or quilt, and covered with a layer of plastic film when the slurry is collected. Before form removal, cover and spray water for curing. Sprinkler curing is carried out by automatic sprinkler system and sprayer, and wet curing is uninterrupted. The watering interval is determined according to the air temperature, and the surface is kept moist continuously before formwork removal. After form removal, the curing process of spraying curing agent and covering with plastic sheets should be adopted, and the newly poured concrete exposed to the atmosphere should be sprayed with curing agent in time. When the newly poured structure is easy to contact with running water, measures should be taken to prevent concrete from being washed by water within 1d after pouring.
6. In the process of concrete curing, the temperature sensor is buried in the concrete, and an automatic temperature monitor is used to compile a temperature monitoring program to automatically alarm and monitor the internal temperature, curing temperature and ambient temperature of the concrete. When the temperature is higher than 40℃, immediately start the cooling system to reduce the temperature to prevent the internal temperature of concrete from being higher than 55℃.
7. When dismantling the formwork of bearing platform, pier and cast-in-place beam, in order to prevent concrete from cracking due to excessive temperature difference between inside and outside, the internal temperature and ambient temperature of concrete must be tested in advance, and the formwork can only be dismantled when the difference between the internal temperature and ambient temperature of concrete is lower than 15℃.
8. Strictly control and check the construction of concrete protective layer. All concrete structures adopt customized plastic protective layer positioning clips (blocks) with reliable performance, and their sizes and shapes meet the design requirements. Up and down spacing is not more than 40cm, left and right spacing is not more than 50cm. They are arranged in a plum blossom shape and are firmly fixed. Only positive deviation is allowed for the thickness of the protective layer of concrete structure, and negative deviation is never allowed. The thickness detector of the protective layer of steel bar is used for comprehensive detection.
9. In the specific construction process, establish continuous meteorological forecasting and observation systems, institutions and systems. And there are specialized agencies and personnel responsible for temperature detection, monitoring and maintenance. Find problems and take timely measures.
10. Establish perfect and reliable emergency measures and stand by at any time.
1.4 technical measures for crack prevention of pile cap concrete
1. The pouring of the mass concrete of the bearing platform should be carried out when the temperature is low in a day, and the thickness of the pouring layer should be reduced, the pouring time and heat dissipation time should be prolonged, and it should be divided into several thinner pouring layers, and the thickness of each layer should not be greater than 1.5m, so as to speed up the heat dissipation of the concrete; Cover the concrete material to avoid sun exposure, mix the concrete with cooling water, and reduce the warehousing temperature within the range of 10 ~ 15℃, preferably around 1℃; Burying the cooling steel pipe into concrete for water cooling; Immediately after pouring, technical measures such as spray curing, covering, heat preservation, etc. are taken, and it is always in a wet curing state, and a shade shed is set up in high temperature season in summer. The curing time shall not be less than 10 day.
2. The quantity, setting mode, cooling water temperature and cooling process of embedded cooling steel pipes for mass concrete of the bearing platform must be model tested before formal pouring to determine reasonable cooling construction process parameters.
1.5 technical measures for crack prevention of pier concrete
The maintenance of pier concrete is to wrap a quilt on the formwork first, and then build a tower from bottom to top outside the formwork. The tower is firmly surrounded by a quilt and a canvas, and the bottom and top are covered with a sealed heat insulation layer. In summer, a water spraying system is set between the formwork and the tower, and cooling water is sprayed to reduce the temperature, so that the formwork temperature and the surrounding temperature do not exceed 30℃. After form removal, the curing process of spraying curing agent and covering with plastic sheets shall be adopted, and the newly poured concrete exposed to the atmosphere shall be sprayed with curing agent in time. The internal temperature, curing temperature and ambient temperature of concrete are monitored by an automatic temperature monitor.
1.6 technical measures for crack prevention of cast-in-place beam concrete
1.6. 1 Basic measures for crack prevention of cast-in-place beam concrete
1. Take maintenance measures to wrap a layer of quilt and canvas on both sides of the cast-in-place beam concrete maintenance template, set up steel pipe racks at both ends outward and wrap a layer of quilt and canvas, and generally take sprinkler maintenance measures to cover linen sheets or quilts and plastic films at the top. When the temperature is high in summer, build a shade shed at the top to prevent direct sunlight from hitting the concrete. Spray curing agent immediately after mold removal. Temperature sensors are buried in different parts of concrete, and the internal temperature, curing temperature and environmental temperature of concrete are automatically alarmed and monitored by using automatic temperature monitor.
2. When the cast-in-place beam formwork is dismantled, the temperature difference between the concrete core and the surface, inside and outside the box, and between the surface and the environment shall not be greater than 65438 05℃.
3. The tensioning sequence of longitudinal prestressed tendons of cast-in-place beams is four jacks, symmetrical from left to right, balanced from top to bottom, first in the middle and then on both sides, so as to ensure the stress balance of the whole tensioning end face and prevent the box beam from cracking due to unbalanced stress.
1.6.2 technical measures for crack prevention of cantilever beam
1. In order to prevent the cracking of the cantilever beam block 0, the bracket of block 0 adopts the floor-standing bracket scheme, that is, the bracket is supported on the concrete bearing platform, and the whole bracket system is preloaded with equal load by jack to eliminate inelastic deformation, and the measured elastic deformation is used as the basis of construction allowance. When pouring concrete, the anti-cracking technology of equal load preloading and equivalent load reduction with concrete pouring quantity developed by our company is adopted.
2. The prestressed tensioning stage is divided into two stages, namely:
The first stage is early tensioning. When the concrete strength reaches about 50% of the design strength, only the end formwork, the side formwork and the internal formwork are loosened, and then the early tensioning method is carried out to prevent the cast-in-place beam from drying shrinkage or temperature cracks. The tension quantity and value of prestressed tendons are determined by calculation, and the tension is generally between 40% and 50% of the design value.
The second stage is final tensioning. When the concrete strength and elastic modulus reach more than 80% of the design strength, all the formwork will be removed and all the prestressed tendons will be tensioned to the design tonnage.
3. Rigid locking of closure section and concrete pouring must be carried out when the temperature is low and the temperature is the lowest in a day. After rigid locking, partial prestress should be applied to the through prestressed tendons in time to ensure that the closed section is not affected by concrete deformation.
1.6.3 technical measures for crack prevention of cast-in-place beams by support method and moving formwork method
1. The pouring of cast-in-place beam shall be carried out when the temperature is low in a day. When pouring concrete, the construction scheme that two delivery pumps and two distributing rods are carried out simultaneously from the mid-span part of the beam to both ends is adopted to eliminate the cracking of the beam caused by mid-span deformation.
2. Change the prestress tensioning stage from two-stage tensioning to three-stage tensioning, namely:
The first stage is the early pretensioning method. When the concrete strength reaches about 40% of the design strength, only the end formwork, side formwork and internal formwork are loosened, and then the early pretensioning method is carried out to prevent the cast-in-place beam from drying shrinkage or temperature cracks. The tension quantity and value of prestressed tendons are determined by calculation, and the tension is generally between 30% and 40% of the design value.
The second stage is initial tensioning, the concrete strength reaches more than 60% of the design strength, and the prestressed tendons are tensioned to 60 ~ 10% of the design value.
The third stage is final tensioning. When the concrete strength and elastic modulus reach more than 80% of the design strength, all the formwork will be removed and all the prestressed tendons will be tensioned to the design tonnage.
1.6.4 technical measures for crack prevention of steel-concrete composite beams
1. Concrete pouring shall be carried out as far as possible in a day when the temperature is low, and shall be carried out continuously, once before the initial setting time.
2. The concrete pouring sequence should be strictly in accordance with the design requirements, that is, the concrete in the positive bending distance area should be poured first to form a joint section, and then the concrete in the negative bending distance area should be poured to reduce the bridge deck cracks in the negative bending distance area. First, part of concrete is poured to reach the design strength of 100%, and then part of concrete is poured. The concrete at the joint should be roughened to enhance the binding force of concrete.
3. Concrete pouring should be carried out in sections, plastering should be carried out immediately after each section of pouring, the surface should be closely attached with plastic film, then covered with wet sacks, and finally covered with a layer of canvas to prevent water loss and cracking on the bridge deck surface.
1.7 Precast beam concrete crack prevention technical measures
1. Steam curing: two 2t steam boilers are used for steam heating. Steam curing of precast beams is carried out in a curing shed assembled by large steel skeleton. The steel skeleton is bolted with a 5cm thick rock wool board, which is covered with a canvas and sealed. After concrete pouring, the curing shed immediately moves to the position of precast beam along the slideway, steam pipes are laid on both sides and inside the box girder, and both ends are covered and sealed with double quilts, that is, canvas. Steam curing of precast beams can be divided into four stages: static stop, heating, constant temperature and cooling. During static stop, the temperature in the shed should be kept above 5℃, and the temperature should be increased after 4 hours of pouring, and the heating rate should be controlled at 6-8℃/h. At constant temperature, the temperature of beam core concrete should not exceed 60℃ and the cooling rate should not exceed 10℃. During the whole curing process, the temperature difference between beam core and surface layer, surface layer and environment shall not exceed 65438 05℃.
2. Temperature monitoring: During the whole curing process, three multipoint automatic alarm thermometers are used to monitor the temperature of the beam core, surface layer and environment respectively. The maximum alarm temperature of the core thermometer is set to 55℃, the minimum alarm temperature is set to 40℃, the maximum alarm temperature of the surface and environmental thermometers is set to 50℃, and the minimum alarm temperature is set to 45℃. When the temperature difference between the beam core and the surface layer, and between the surface layer and the environment exceeds 65438 05, after the steam curing, it will immediately enter the natural curing.
3. Natural conservation: Natural conservation uses straw bags or sacks to cover water, and plastic films are covered on them for conservation. After form removal, the curing process of spraying curing agent and covering plastic sheets shall be adopted, and the curing agent shall be sprayed on the newly poured concrete exposed to the atmosphere in time. The difference between the curing agent sprayed on concrete and the surface temperature of concrete should not be greater than 15℃, and the curing agent should meet the requirements of Cement Concrete Curing Agent (JC90 1-2002).
4. The tensioning sequence of prestressed tendons is four jacks, symmetrical from left to right, balanced from top to bottom, first in the middle and then on both sides, so as to ensure the stress balance of the whole tensioning end face and prevent the box beam from cracking due to unbalanced stress.
5. Prestress tensioning should be carried out as soon as possible, and the prestress tensioning stage is divided into three stages, namely:
The first stage is the early pretensioning method. When the concrete strength reaches about 50% of the design strength, only the end formwork, side formwork and internal formwork are loosened, and then the early pretensioning method is carried out to prevent the cast-in-place beam from drying shrinkage or temperature cracks. The tensioning quantity and value of prestressed tendons shall be carried out according to the design requirements, and the tensioning force is generally between 30% and 40% of the design value.
The second stage is initial tensioning. When the concrete strength reaches more than 80% of the design strength, the formwork will be removed and the prestressed tendons will be tensioned according to the design requirements.
The third stage is final tensioning. When the concrete strength and elastic modulus reach more than 100% of the design strength and the age is not less than 10 days, all prestressed tendons are tensioned to the design tonnage.
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